Importer helper command behaviour can vary

GitUbuntuRepository:quilt_env currently doesn't make any arrangement to use "--quiltrc -" and nor do any of its callers. This means that the imported result (especially applied branches) may vary depending on what the user has in /etc/quiltrc or ~/.quiltrc.

We should ensure that all QUILT_* environment variables are exactly as we want them and "--quiltrc -" is used.

This also affects gbp via repository, user, and global config options. The former is especially annoying because, e.g., src:at sets a gbp.conf that enables pbuilder which then causes our gbp-buildpackage (which, admittedly, we will switch to export-orig) command to error out as it tries to use cowbuilder... In my testing so far, setting GBP_CONF_FILES=/dev/null is a suitable workaround. I have requested (via e-mail) upstream consider allowing an empty string value, as that's a slightly more clear semantic.